MIT's guitar combines DSP with interchangeable sound

PORTLAND, Ore.  Guitarists revel in the diverse acoustic resonances emanating from instruments made from different types of wood. As a result, most guitarists own a variety of guitars.

Now, researchers at the Massachusetts Institute of Technology (MIT) have combined digital signal processing with a removable sound board, enabling the Chameleon guitar to change its sonic characteristics by changing the type of wood used for the sound board. The design also retains the same guitar body, neck and frets.

"We have made the acoustic soundboard of a guitar removable so you can exchange it with different woods to change the way it resonates," said MIT Media Lab's Amit Zoran, inventor of the Chameleon Guitar. "Each sound board has five piezoelectric sensors [used] to capture acoustic vibrations. Then we use digital signal processing to make it sound like its coming from a full-sized chamber on a normal acoustic guitar."

The size of the sound board is only about 30 percent of a full-sized acoustic guitar sound board, which would ordinarily sacrifice low-frequency resonances. Using signal processing software to compensate, Zoran claimed the acoustic subtleties of the guitar rival a full-sized guitar. The result is a compact guitar, but with acoustic properties that can be modified by exchanging sound boards.

"Now the player can modify the sound of his guitar using the acoustic properties of real wood, rather than just use a synthesizer to simulate it," said Zoran.

Zoran first used linear analog processing, summing the signals from the five piezoelectric transducers, enhancing the lower frequencies while attenuating the natural resonant frequency of the sound board itself.

For his second-generation Chameleon prototype, Zoran plans to use a Freescale Symphony DSP56371 device and the chip maker's Symphony SoundBite Development Kit. "The idea is to try different digital processing approaches, including linear filtering, finite element-based approaches and sound synthesis-based methods to see which works the best," said Zoran.

Zoran has been experimenting with different woods and other materials for the Chameleon guitar sound board. The most exotic board came from a piece of 150-year-old spruce from a Vermont bridge. He is also working with instrument builder Marco Coppiardi of Boston-based Italiastrings to develop a sound board that can be easily swapped in and out.